1. Field of the Invention
The present invention relates to a high zoom-ratio zoom lens system that is suitable for an imaging optical system of a digital camera that is provided with a small (compact) image sensor.
2. Description of Related Art
In recent years, technical advancement in providing higher zoom ratios has occurred in zoom lens systems, while the demand for further miniaturization in zoom lens systems due to further miniaturization of the camera body has also increased. For example, zoom lens systems having five lens groups, i.e., a positive lens group, a negative lens group, a positive lens group, a negative lens group and a positive lens group (Japanese Unexamined Patent Publication Nos. 2009-244443 and 2009-175324), and zoom lens systems having four lens groups, i.e., a positive lens group, a negative lens group, a positive lens group and a positive lens group (Japanese Unexamined Patent Publication Nos. 2009-58980 and 2005-331697), are known in the art as relatively high zoom-ratio zoom lens systems.
However, in zoom lens systems having five lens groups (i.e., a positive lens group, a negative lens group, a positive lens group, a negative lens group and a positive lens group) as disclosed in the above-mentioned Japanese Unexamined Patent Publication Nos. 2009-244443 and 2009-175324, due to being such zoom lens systems of five lens groups, a problem occurs with an increase in the size of the zoom lens system becoming larger due to an increased number of components and further complexity of the mechanical structures therefor, which makes it very difficult to achieve a compact zoom lens system.
Furthermore, in zoom lens systems having four lens groups (i.e., a positive lens group, a negative lens group, a positive lens group and a positive lens group), disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 2009-58980, the overall length of the zoom lens system at the short focal length extremity is great; and, the zoom lens system is not sufficiently miniaturized. Furthermore, in the zoom lens system disclosed in the above-mentioned Japanese Unexamined Patent Publication No. 2005-331697, although miniaturization is achieved, the focal length at the long focal length extremity is 120 mm and the zoom ratio is 6.4:1, which is somewhat small.